In the field of wireless power transfer, inductive coupling has been used to provide power to and communicate with a device without making electrical contact. This technique has been used, for example, with implanted medical devices. Systems utilizing this technique have an external unit that is a power transmitter and a medical device implanted within the body of a patient that is a power receiving unit. A coil driver applies an AC signal to a primary coil in the external unit, generating a magnetic field. The power transmitter is placed in proximity to the body of the patient so that the magnetic field induces a current on a secondary coil in the implanted medical device. A power management unit in the implant can use the current induced on the secondary coil to charge a battery or to directly operate the implanted medical device. To provide communication between the coils, the power signal on the secondary coil is load modulated by a modulator. This modulation is picked up by a demodulator attached to the primary coil. Using this method, systems communicate and transmit power on a single inductive link simultaneously.
In these inductive power transfer and communication systems, the coils are susceptible to parasitic capacitances and parasitic conductances. In particular, parasitic variations can be introduced by the presence of tissue near the coils, a circumstance which is presented frequently with implanted medical devices when the external unit is handled or when it is placed near the target implant. Parasitic variations may also be introduced by conductive surfaces which cut across the magnetic field generated by the primary coil. These parasitic variations can alter the inductive link between the coils, reducing the efficiency of power transfer or interfering with the communication of data. To address the changes in operation of the inductive link caused by parasitic variations, prior art systems have used frequency shifting or active re-tuning. See Troyk, U.S. Pat. No. 5,179,511; Stover, U.S. Pat. No. 7,190,153. These solutions may address the parasitic variations, but prevent the operation of the system at very fixed frequencies. For regulatory reasons, the use of some compliant technologies (such as near field communication, regulated under ISO/IEC 18092) requires operation at very fixed frequencies.
The coils in inductive power transfer and communication systems usually operate with large currents and/or voltages. Accordingly the modulation elements and demodulation elements applied to these coils need to be able to handle large currents, large voltages, or both. This generally increases the size of the components used to modulate the power signal and increases the stress levels on the components, and requires the demodulator to tolerate a large input signal. This can add to the weight and cost of the device, and reduce the longevity of the device.
Accordingly, there is an ongoing need for inductive power transfer and communication systems that are resistant or immune to parasitic variations introduced externally and that accomplish modulation and demodulation with lesser demands on the modulation and demodulation components.